Implemented M_RatioReduce - using Euclid's Algorithm reduce the given…

… numerator and denominator by their greatest common divisor.

doomsday/engine/api/doomsday.def

@@ -1,6 +1,6 @@
 ; Doomsday Engine API (Routines exported from Doomsday.exe).
 ;
-; Highest ordinal is currently: --> 498 <--
+; Highest ordinal is currently: --> 499 <--
 ; Other free ordinals: 30
 
 NAME                            "DOOMSDAY"
@@ -538,6 +538,7 @@ EXPORTS
     M_RunTrigger                @341 NONAME
     M_CeilPow2                  @422 NONAME
     M_NumDigits                 @143 NONAME
+    M_RatioReduce               @499 NONAME
 
 ; Math.
     ; Binary angles.

doomsday/engine/api/doomsday.h

@@ -107,7 +107,7 @@ gameid_t DD_AddGame(const char* identityKey, const char* dataPath, const char* d
  * @param rflags        @see resourceFlags
  * @param names         One or more known potential names, seperated by semicolon e.g., "name1;name2".
  *                      Names may include valid absolute, or relative file paths. These paths include
- *                      valid symbolbolic escape tokens, predefined symbols into the virtual file system.            
+ *                      valid symbolbolic escape tokens, predefined symbols into the virtual file system.
  */
 void DD_AddGameResource(gameid_t game, resourceclass_t rclass, int rflags, const char* names, void* params);
 
@@ -531,6 +531,7 @@ void Con_SetString(const char* name, char* text);
     int             M_ScreenShot(const char* filename, int bits);
     int             M_CeilPow2(int num);
     int             M_NumDigits(int value);
+    int             M_RatioReduce(int* numerator, int* denominator);
 
     // MiscellaneousL: Time utilities.
     boolean         M_RunTrigger(trigger_t* trigger, timespan_t advanceTime);

doomsday/engine/portable/include/m_misc.h

@@ -135,6 +135,16 @@ int             M_FloorPow2(int num);
 int             M_RoundPow2(int num);
 int             M_WeightPow2(int num, float weight);
 float           M_CycleIntoRange(float value, float length);
+
+/**
+ * Using Euclid's Algorithm reduce the given numerator and denominator by
+ * their greatest common integer divisor.
+ * @param numerator  Input and output numerator.
+ * @param denominator  Input and output denominator.
+ * @return  Greatest common divisor.
+ */
+int M_RatioReduce(int* numerator, int* denominator);
+
 double          M_SlopeToAngle(double dx, double dy);
 double          M_Length(double x, double y);
 int             M_NumDigits(int num);

doomsday/engine/portable/src/m_misc.c

@@ -341,6 +341,40 @@ void RNG_Reset(void)
     rngIndex = 0, rngIndex2 = 0;
 }
 
+int M_RatioReduce(int* numerator, int* denominator)
+{
+    assert(numerator && denominator);
+    {
+    int n, d, temp;
+
+    if(*numerator == *denominator)
+        return 1; // 1:1
+
+    n = abs(*numerator);
+    d = abs(*denominator);
+    // Ensure numerator is larger.
+    if(n < d)
+    {
+        temp = n;
+        n = d;
+        d = temp;
+    }
+
+    // Reduce to the common divisor.
+    while(d != 0)
+    {
+        temp = n;
+        n = d;
+        d = temp % d;
+    }
+
+    // Apply divisor.
+    *numerator   /= n;
+    *denominator /= n;
+    return n;
+    }
+}
+
 /**
  * Finds the power of 2 that is equal to or greater than the specified
  * number.